Ruqing Wang

The presence of heterogeneous media can produce significant perturbations of dose distribution in brachytherapy. In a companion paper, we proposed a dose decomposition approach for dose calculation in a heterogeneous medium, which separately treats dose contributions from primary, once-scattered and multiple-scattered photons. The companion paper also describes and verifies a micro-beam ray-tracing method for evaluating the once-scatter dose. This paper deals with the calculation of the multiple-scatter dose. We present two empirical formulations for evaluating the heterogeneity correction factor for a 27 keV point source in a water sphere containing a disc-shaped heterogeneity. The empirical formulations are based on nonlinear curve fitting of the Monte Carlo multiple-scatter dose estimates calculated for the heterogeneous system. Extensive benchmark comparisons show that these formulations provide results for the multiple-scatter dose that agree within 10% (and mostly within 5%) with corresponding Monte Carlo dose estimates. Combining them with the algorithms for primary and once-scatter dose calculation described in the companion paper yields results for the total dose of equivalent accuracy. The empirical formulations are expressed in simple mathematical forms which involve a separation of the geometry and position variables of the heterogeneous system. Such representation provides a good tool to investigate the heterogeneity-induced perturbation of a multiple-scatter dose at low photon energy.

In this work, we propose a framework for calculating brachytherapy dose distributions in heterogeneous media. The approach taken includes analytical calculation of the primary dose, and separately treats contributions of the once-scatter photons and multiple-scatter photons to the total scatter dose. This paper focuses on the evaluation of the once-scatter dose, which is based on a micro-beam ray-tracing model developed by the authors that incorporates an accurate description of the physical scattering of photons (Compton and Rayleigh scattering) with considerable flexibility in accommodating diverse geometries in a heterogeneous medium. The accuracy of the ray-tracing model has been verified by comparing model-calculated once-scatter doses with corresponding Monte Carlo results. For a 22 keV, 27 keV and 300 keV point source in water containing a disc-shaped heterogeneity of whitlockite, stainless steel or lead, our calculated results for once-scatter doses are in excellent agreement with corresponding Monte Carlo results over a wide range of heterogeneity dimensions and positions. Our investigation also explores the differences between physical scattering and isotropic scattering in evaluating the once-scatter dose, and thus enables the domain of applicability of the latter to be assessed. An appropriate method for evaluating the multiple-scatter dose, which together with the micro-beam method described here provides a means to calculate the total dose, is the subject of a companion paper.

A new ⁹⁰Sr/⁹⁰Y catheter-based system, the Beta-Cath™ 3.5F system, was recently introduced into the clinic for intravascular brachytherapy. This new system has a smaller diameter of delivery catheter (3.5 French) compared to the original Beta-Cath™ system (5 French catheter). The new source/seed (3.5F source) has a different design from the original (5F) source. In this paper, we report the EGSnrc Monte Carlo results of two-dimensional dose rate distributions in water as well as the TG-60 dose parameters for the new 3.5F source and the original 5F source. A complete set of the dosimetric data for each individual seed is tabulated. A detailed dosimetric comparison was carried out for the two sources. The comparison shows that differences between the two sources are less than 5% in the region of interest for intravascular brachytherapy. Higher differences (up to 10%) are seen in the near-source region, which is mainly inside the delivery catheter. Based on the present calculation and comparison, we conclude that the new Beta-Cath™ 3.5F system is dosimetrically equivalent to the original Beta-Cath™ (5F) system. Therefore, clinical dosimetry and the practice of using both the systems can be treated similarly.

The purpose of the work is to calculate basic dosimetry data for a VariSource high dose rate 192Ir source in water. These basic dosimetry data, expressed in the dose calculation formalism endorsed by the Interstitial Collaborative Working Group and AAPM Task group 43, include the dose rate constant, the radial dose function, and the anisotropy function. A modified version of the EGS4 Monte Carlo code was used to calculate (1) the transverse-axis dose distribution at radial distances from 0.1 to 14 cm, (2) the two-dimensional dose distribution for axial and radial distances from 0.1 cm to 10 cm, and (3) the air-kerma strength, for the VariSource high dose rate 192Ir source. From these Monte Carlo results the basic dosimetry data were derived. The calculated dose rate constant for the high dose rate source is 1.044 +/- 0.2% cGy h-1 per unit air-kerma strength. The anisotropy function exhibits 40%-60% deviations from isotropy at positions on the long axis. The radial dose function for the source is nearly identical to that for a microSelectron high dose rate 192Ir source, except at radial distances smaller than 0.5 cm where values for VariSource are 1.7%-2.8% smaller. These basic dosimetry data were compared with corresponding results from other authors for high and low dose rate 192Ir sources, as well as with Meisberger's fitting formula.

Computer algorithms for electron binding correction to Compton scattering and for detailed simulation of K-edge characteristic x-ray production were incorporated into EGS4 unix version 2.0. Based on detailed modelling of the internal structures of sources, the modified version was used to calculate dose rate constants, radial dose functions, and anisotropy functions on the long axis for an 125I model 6711 source, 169Yb Type 5 and Type 8 sources, and a stainless steel clad (SS) 192Ir source. The geometry of these sources is cylindrically symmetric. Calculated results are generally in good agreement with corresponding values recommended by TG-43 and Monte Carlo results published by other authors. The influence of electron binding in Compton scattering on the calculated dose distribution for an 125I model 6711 source in water, and of different characteristic x-ray production models for 125I model 6711 and 192Ir SS sources, were also studied.